Imprint lithography

ABSTRACT

A lithographic apparatus is disclosed that has a template holder configured to hold an imprint template, a substrate table arranged to receive a substrate, a radiation output arranged to illuminate a part of the imprint template, and a detector configured to detect radiation scattered from an interface between the imprint template and imprintable material provided on the substrate.

1. FIELD

The invention relates to imprint lithography.

2. BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus areconventionally used, for example, in the manufacture of integratedcircuits (ICs), flat panel displays and other devices involving finestructures.

It is desirable to reduce the size of features in a lithographic patternbecause this allows for a greater density of features on a givensubstrate area. In photolithography, the increased resolution may beachieved by using radiation of shorter wavelength. However, there areproblems associated with such reductions. Current systems are startingto adopt optical sources with wavelengths in the 193 nm regime but evenat this level, diffraction limitations become a barrier. At lowerwavelengths, the transparency of materials is very poor. Opticallithography machines capable of enhanced resolutions require complexoptics and rare materials and are consequently very expensive.

An alternative for printing sub-100 nm features, known as imprintlithography, comprises transferring a pattern to a substrate byimprinting a pattern into an imprintable medium using a physical mouldor template. The imprintable medium may be the substrate or a materialcoated on to a surface of the substrate. The imprintable medium may befunctional or may be used as a “mask” to transfer a pattern to anunderlying surface. The imprintable medium may, for instance, beprovided as a resist deposited on a substrate, such as a semiconductormaterial, into which the pattern defined by the template is to betransferred. Imprint lithography is thus essentially a moulding processon a micrometer or nanometer scale in which the topography of a templatedefines the pattern created on a substrate. Patterns may be layered aswith optical lithography processes so that, in principle, imprintlithography could be used for such applications as IC manufacture.

The resolution of imprint lithography is limited only by the resolutionof the template fabrication process. For instance, imprint lithographymay be used to produce features in the sub-50 nm range withsignificantly improved resolution and line edge roughness compared tothat achievable with conventional optical lithography processes. Inaddition, imprint processes do not require expensive optics, advancedillumination sources or specialized resist materials typically requiredby optical lithography processes.

In a conventional imprint lithography apparatus, a template providedwith a pattern is attached to an actuator. The actuator moves thetemplate towards a substrate, and pushes the template onto thesubstrate. The forces pushing onto the substrate via the template may bequite large. This may cause deformation of the substrate and/ortemplate, for example of a few hundred nanometers, which may lead todamage of the pattern imprinted on the substrate.

In general, a considerable amount of time is allowed to lapse once animprint template has been brought into contact with the imprintablematerial of a substrate. This is to allow the imprintable material tofully flow into all of the recesses of the pattern of the template. Thisconsiderable time period is one reason why imprint lithography iscurrently slower than optical lithography. Speed is an important factorin the economic viability of imprint lithography machines, and theconsiderable time allowed for the fluid to flow into the template may bea disadvantage.

3. SUMMARY

According to a first aspect, there is provided a lithographic apparatus,comprising:

a template holder configured to hold an imprint template;

a substrate table arranged to receive a substrate;

a radiation output arranged to illuminate a part of the imprinttemplate; and

a detector configured to detect radiation scattered from an interfacebetween the imprint template and imprintable material provided on thesubstrate.

According to a second aspect, there is provided a lithographicapparatus, comprising:

a template holder configured to hold an imprint template;

a substrate table arranged to receive a substrate;

a radiation output arranged to illuminate a part of the imprinttemplate; and

a detector configured to detect radiation reflected by the substrate.

According to a third aspect, there is provided a method of imprintlithography, comprising:

pressing an imprint template onto a layer of imprintable material on asubstrate;

illuminating a part of the imprint template with radiation; and

detecting radiation scattered from an interface between the imprinttemplate and the imprintable material on the substrate.

According to a fourth aspect, there is provided a method of imprintlithography, comprising:

pressing an imprint template onto a layer of imprintable material on asubstrate;

illuminating a part of the imprint template with radiation; and

detecting radiation reflected by the substrate.

One or more embodiments of the invention are applicable to any imprintlithography process in which a patterned template is imprinted into animprintable medium in a flowable state, and for instance can be appliedto hot and UV imprint lithography as described herein.

4. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 a-1 c illustrate examples of conventional soft, hot and UVlithography process respectively;

FIG. 2 a-2 c illustrate a two step etching process employed when hot andUV imprint lithography is used to pattern a resist layer;

FIG. 3 schematically illustrates a template and a typical imprintableresist layer deposited on a substrate;

FIG. 4 schematically illustrates an imprint lithography apparatusaccording to an embodiment of the invention;

FIG. 5 a-5 c schematically depict details of the imprint template andsubstrate shown in FIG. 4; and

FIG. 6 schematically illustrates an imprint lithography apparatusaccording to another embodiment of the invention.

5. DETAILED DESCRIPTION

There are two principal approaches to imprint lithography which will betermed generally as hot imprint lithography and UV imprint lithography.There is also a third type of “printing” lithography known as softlithography. Examples of these are illustrated in FIGS. 1 a to 1 c.

FIG. 1 a schematically depicts the soft lithography process whichinvolves transferring a layer of molecules 11 (typically an ink such asa thiol) from a flexible template 10 (typically fabricated frompolydimethylsiloxane (PDMS)) onto a resist layer 13 which is supportedupon a substrate 12 and planarization and transfer layer 12′. Thetemplate 10 has a pattern of features on its surface, the molecularlayer being disposed upon the features. When the template is pressedagainst the resist layer, the layer of molecules 11 stick to the resist.Upon removal of the template from the resist, the layer of molecules 11stick to the resist, the residual layer of resist is etched such thatthe areas of the resist not covered by the transferred molecular layerare etched down to the substrate.

The template used in soft lithography may be easily deformed and maytherefore not be suited to high resolution applications, e.g. on ananometer scale, since the deformation of the template may adverselyaffect the imprinted pattern. Furthermore, when fabricating multiplelayer structures, in which the same region will be overlaid multipletimes, soft imprint lithography may not provide overlay accuracy on ananometer scale.

Hot imprint lithography (or hot embossing) is also known as nanoimprintlithography (NIL) when used on a nanometer scale. The process uses aharder template made from, for example, silicon or nickel, which aremore resistant to wear and deformation. This is described for instancein U.S. Pat. No. 6,482,742 and illustrated in FIG. 1 b. In a typical hotimprint process, a solid template 14 is imprinted into a thermosettingor a thermoplastic polymer resin 15, which has been cast on the surfaceof substrate. The resin may, for instance, be spin coated and baked ontothe substrate surface or more typically (as in the example illustrated)onto a planarization and transfer layer 12′. It should be understoodthat the term “hard” when describing an imprint template includesmaterials which may generally be considered between “hard” and “soft”materials, such as for example “hard” rubber. The suitability of aparticular material for use as an imprint template is determined by itsapplication requirements.

When a thermosetting polymer resin is used, the resin is heated to atemperature such that, upon contact with the template, the resin issufficiently flowable to flow into the pattern features defined on thetemplate. The temperature of the resin is then increased to thermallycure (e.g. crosslink) the resin so that it solidifies and irreversiblyadopts the desired pattern. The template may then be removed and thepatterned resin cooled.

Examples of thermoplastic polymer resins used in hot imprint lithographyprocesses are poly(methyl methacrylate), polystyrene, poly(benzylmethacrylate) or poly(cyclohexyl methacrylate). The thermoplastic resinis heated so that it is in a freely flowable state immediately prior toimprinting with the template. It is typically necessary to heatthermoplastic resin to a temperature considerably above the glasstransition temperature of the resin. The template is pressed into theflowable resin and sufficient pressure is applied to ensure the resinflows into all the pattern features defined on the template. The resinis then cooled to below its glass transition temperature with thetemplate in place whereupon the resin irreversibly adopts the desiredpattern. The pattern will consist of the features in relief from aresidual layer of the resin which may then be removed by an appropriateetch process to leave only the pattern features.

Upon removal of the template from the solidified resin, a two-stepetching process is typically performed as illustrated in FIGS. 2 a to 2c. The substrate 20 has a planarization and transfer layer 21immediately upon it, as shown in FIG. 2 a. The purpose of theplanarization and transfer layer is twofold. It acts to provide asurface substantially parallel to that of the template, which helpsensure that the contact between the template and the resin is parallel,and also to improve the aspect ratio of the printed features, asdescribed herein.

After the template has been removed, a residual layer 22 of thesolidified resin is left on the planarization and transfer layer 21,shaped in the desired pattern. The first etch is isotropic and removesparts of the residual layer 22, resulting in a poor aspect ratio offeatures where L1 is the height of the features 23, as shown in FIG. 2b. The second etch is anisotropic (or selective) and improves the aspectratio. The anisotropic etch removes those parts of the planarization andtransfer layer 21 which are not covered by the solidified resin,increasing the aspect ratio of the features 23 to (L2/D), as shown inFIG. 2 c. The resulting polymer thickness contrast left on the substrateafter etching can be used as for instance a mask for dry etching if theimprinted polymer is sufficiently resistant, for instance as a step in alift-off process.

Hot imprint lithography suffers from a disadvantage in that not onlymust the pattern transfer be performed at a higher temperature, but alsorelatively large temperature differentials might be required in order toensure the resin is adequately solidified before the template isremoved. Temperature differentials between 35 and 100° C. may be needed.Differential thermal expansion between, for instance, the substrate andtemplate may then lead to distortion in the transferred pattern. Thismay be exacerbated by the relatively high pressure needed for theimprinting step, due the viscous nature of the imprintable material,which can induce mechanical deformation in the substrate, againdistorting the pattern.

UV imprint lithography, on the other hand, does not involve such hightemperatures and temperature changes nor does it require such viscousimprintable materials. Rather, UV imprint lithography involves the useof a partially or wholly transparent template and a UV-curable liquid,typically a monomer such as an acrylate or methacrylate. In general, anyphotopolymerizable material could be used, such as a mixture of monomersand an initiator. The curable liquid may also, for instance, include adimethyl siloxane derivative. Such materials are less viscous than thethermosetting and thermoplastic resins used in hot imprint lithographyand consequently move much faster to fill template pattern features. Lowtemperature and low pressure operation also favors higher throughputcapabilities.

An example of a UV imprint process is illustrated in FIG. 1 c. A quartztemplate 16 is applied to a UV curable resin 17 in a similar manner tothe process of FIG. 1 b. Instead of raising the temperature as in hotembossing employing thermosetting resins, or temperature cycling whenusing thermoplastic resins, UV radiation is applied to the resin throughthe quartz template in order to polymerize and thus cure it. Uponremoval of the template, the remaining steps of etching the residuallayer of resist are the same or similar as for the hot embossing processdescribed herein. The UV curable resins typically used have a much lowerviscosity than typical thermoplastic resins so that lower imprintpressures can be used. Reduced physical deformation due to the lowerpressures, together with reduced deformation due to high temperaturesand temperature changes, makes UV imprint lithography suited toapplications requiring high overlay accuracy. In addition, thetransparent nature of UV imprint templates can accommodate opticalalignment techniques simultaneously to the imprinting.

Although this type of imprint lithography mainly uses UV curablematerials, and is thus generically referred to as UV imprintlithography, other wavelengths of radiation may be used to cureappropriately selected materials (e.g., activate a polymerization orcross linking reaction). In general, any radiation capable of initiatingsuch a chemical reaction may be used if an appropriate imprintablematerial is available. Alternative “activating radiation” may, forinstance, include visible light, infrared radiation, x-ray radiation andelectron beam radiation. In the general description herein, referencesto UV imprint lithography and use of UV radiation are not intended toexclude these and other activating radiation possibilities.

As an alternative to imprint systems using a planar template which ismaintained substantially parallel to the substrate surface, rollerimprint systems have been developed. Both hot and UV roller imprintsystems have been proposed in which the template is formed on a rollerbut otherwise the imprint process is very similar to imprinting using aplanar template. Unless the context requires otherwise, references to animprint template include references to a roller template.

There is a particular development of UV imprint technology known as stepand flash imprint lithography (SFIL) which may be used to pattern asubstrate in small steps in a similar manner to optical steppersconventionally used, for example, in IC manufacture. This involvesprinting small areas of the substrate at a time by imprinting a templateinto a UV curable resin, ‘flashing’. UV radiation through the templateto cure the resin beneath the template, removing the template, steppingto an adjacent region of the substrate and repeating the operation. Thesmall field size of such step and repeat processes may help reducepattern distortions and CD variations so that SFIL may be particularlysuited to manufacture of IC and other devices requiring high overlayaccuracy.

Although in principle the UV curable resin can be applied to the entiresubstrate surface, for instance by spin coating, this may be problematicdue to the volatile nature of UV curable resins.

One approach to addressing this problem is the so-called ‘drop ondemand’ process in which the resin is dispensed onto a target portion ofthe substrate in droplets immediately prior to imprinting with thetemplate. The liquid dispensing is controlled so that a predeterminedvolume of liquid is deposited on a particular target portion of thesubstrate. The liquid may be dispensed in a variety of patterns and thecombination of carefully controlling liquid volume and placement of thepattern can be employed to confine patterning to the target area.

Dispensing the resin on demand as mentioned is not a trivial matter. Thesize and spacing of the droplets are carefully controlled to ensurethere is sufficient resin to fill template features while at the sametime minimizing excess resin which can be rolled to an undesirably thickor uneven residual layer since as soon as neighboring drops touch theresin will have nowhere to flow.

Although reference is made herein to depositing UV curable liquids ontoa substrate, the liquids could also be deposited on the template and ingeneral the same techniques and considerations will apply.

FIG. 3 illustrates the relative dimensions of the template, imprintablematerial (curable monomer, thermosetting resin, thermoplastic, etc) andsubstrate. The ratio of the width of the substrate, D, to the thicknessof the curable resin layer, t, is of the order of 10⁶. It will beappreciated that, in order to avoid the features projecting from thetemplate damaging the substrate, the dimension t should be greater thanthe depth of the projecting features on the template.

The residual layer left after stamping is useful in protecting theunderlying substrate, but as mentioned herein it may also be the sourceof a problem, particularly when high resolution and/or overlay accuracyis required. The first ‘breakthrough’ etch is isotropic (non-selective)and will thus to some extent erode the features imprinted as well as theresidual layer. This may be exacerbated if the residual layer is overlythick and/or uneven. This problem may, for instance, lead to variationin the thickness of lines ultimately formed in the underlying substrate(i.e. variation in the critical dimension). The uniformity of thethickness of a line that is etched in the transfer layer in the secondanisotropic etch is dependant upon the aspect ratio and integrity of theshape of the feature left in the resin. If the residual resin layer isuneven, then the non-selective first etch can leave some of thesefeatures with “rounded” tops so that they are not sufficiently welldefined to ensure good uniformity of line thickness in the second andany subsequent etch process. In principle, the above problem may bereduced by ensuring the residual layer is as thin as possible but thiscan require application of undesirably large pressures (possiblyincreasing substrate deformation) and relatively long imprinting times(possibly reducing throughput).

The template is a significant component of the imprint lithographysystem. As noted herein, the resolution of the features on the templatesurface is a limiting factor on the attainable resolution of featuresprinted on the substrate. The templates used for hot and UV lithographyare generally formed in a two-stage process. Initially, the desiredpattern is written using, for example, electron beam writing, to give ahigh resolution pattern in resist. The resist pattern is thentransferred into a thin layer of chrome which forms the mask for thefinal, anisotropic etch step to transfer the pattern into the basematerial of the template. Other techniques such as for example ion-beamlithography, X-ray lithography, extreme UV lithography, epitaxialgrowth, thin film deposition, chemical etching, plasma etching, ionetching or ion milling could be used. Generally, a technique capable ofvery high resolution will be used as the template is effectively a 1×mask with the resolution of the transferred pattern being limited by theresolution of the pattern on the template.

The release characteristics of the template may also be a consideration.The template may, for instance, be treated with a surface treatmentmaterial to form a thin release layer on the template having a lowsurface energy (a thin release layer may also be deposited on thesubstrate).

Another consideration in the development of imprint lithography is themechanical durability of the template. The template may be subjected tolarge forces during stamping of the resist, and in the case of hotlithography, may also be subjected to extremes of pressure andtemperature. This may cause wearing of the template, and may adverselyaffect the shape of the pattern imprinted upon the substrate.

In hot imprint lithography, there is a potential advantage in using atemplate of the same or similar material to the substrate to bepatterned in order to reduce differential thermal expansion between thetwo. In UV imprint lithography, the template is at least partiallytransparent to the activation radiation and accordingly quartz templatesare used.

Although specific reference may be made in this text to the use ofimprint lithography in the manufacture of ICs, it should be understoodthat imprint apparatus and methods described may have otherapplications, such as the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories, hard discmagnetic media, flat panel displays, thin-film magnetic heads, etc.

While in the description herein, particular reference has been made tothe use of imprint lithography to transfer a template pattern to asubstrate via an imprintable resin effectively acting as a resist, insome circumstances the imprintable material may itself be a functionalmaterial, for instance having a functionally such as conductivity,optical linear or non-linear response, etc. For example, the functionalmaterial may form a conductive layer, a semi-conductive layer, adielectric layer or a layer having another desirable mechanical,electrical or optical property. Some organic substances may also beappropriate functional materials. Such applications may be within thescope of the invention.

FIG. 4 shows schematically an imprint lithography apparatus whichcomprises a substrate table 30 and a template holder (not shown for easeof illustration and clarity) holding an imprint template 31. A substrate32 is held on the substrate table 30, and is provided with a region ofimprintable material 33. Only part of the substrate table 30 andsubstrate 32 are shown in FIG. 4 for ease of illustration.

The imprint template 31 is movable in the z-direction (standardCartesian coordinates are marked on FIG. 4), from a disengaged positionin which it is not in contact with the imprintable material 33, to animprint position in which the imprint template 31 is pressed into theimprintable material 33. When the imprint template 31 is in the imprintposition, it causes the imprintable material 33 to flow into recesseswhich form a pattern on the imprint template (the recesses are not shownin FIG. 4). The pattern on the imprint template 31 is therebytransferred to the imprintable material 33.

The imprintable material 33 is illustrated as extending just beyondedges of the imprint template 31. In an arrangement, the imprintablematerial may be provided across the entire upper surface of thesubstrate 32.

A light emitting diode (LED) with a radiation output 34 is provided toone side of the imprint template 31, and is arranged to emit radiationat a wavelength of 400 nanometers in the direction of the imprinttemplate. Alternatively or additionally, radiation may be supplied to aradiation output 34 from a remote radiation source. In an embodiment, Abeam stop 38 is provided on an opposite side of the imprint template 31,and is arranged to trap radiation reflected by the imprint template 31and the substrate 32. A condensing lens 36 is located above the imprinttemplate 31, and is arranged to focus an image of the imprint template31 onto a charge coupled device (CCD) camera 37.

In use, once the imprint template 31 has been moved to the imprintposition, i.e. it is pressing into the imprintable material 33, the LED34 is switched on and directs a beam of radiation 35 at an upper surfaceof the imprint template 31. The beam of radiation 35 passes through theimprint template 31, is reflected from the surface of the substrate 32,passes back through the imprint template and is incident upon the beamstop 38. Part of the radiation beam 35 is scattered at an interfacebetween the imprint template 31 and the imprintable material 33. Thescattered radiation, which is represented by arrows 39, is condensed bythe condenser lens 36 onto the CCD camera 37. The scattered radiation 39is used to determine whether the imprintable material 33 has fullyflowed into the recesses which form the pattern on the underside of theimprint template 31.

FIGS. 5 a-5 c schematically depict details of the imprint template andsubstrate shown in FIG. 4. Referring first to FIG. 5 a, the imprinttemplate 31 is in the disengaged position and is located above thesubstrate 32. The substrate 32 is provided with imprintable material 33.The imprintable material 33 is in the form of two droplets, rather thana continuous layer of equal depth. Droplets of imprintable material 33often occur due to the surface tension of the imprintable material.Indeed, it is often the case that the imprintable material 33 isdeliberately provided in droplets, for example in a configuration whichis believed to speed up the imprint process. The droplets of imprintablematerial 33 are shown as having similar dimensions to recesses 40 whichform the pattern on the underside of the imprint template. However, thisis for ease of illustration only, and in practice the droplets ofimprintable material 33 are usually much larger than the recesses 40.

In FIG. 5 b, the imprint template 31 has been moved to the imprintposition and is pressing onto the imprintable material 33. Over time,this pressure forces the imprintable material 33 to flow into recesses40 in the underside of the imprint template 31. Once this flow has beencompleted, the imprintable material 33 fills all of the recesses 40fully, with the result that when the imprint template 31 is moved to thedisengaged position, a pattern formed by the recesses 40 on theunderside of the imprint template is retained by the imprintablematerial 33. FIG. 5 b shows an intermediate point in the flow. At thisintermediate point, the imprintable material 33 has not fully filled therecesses 40 of the imprint template 31, and regions of gas 42 (gasbubbles) remain in the recesses.

The beam of radiation 35 undergoes scattering when it intercepts theinterface between the gas bubbles 42 and their surroundings. As in FIG.4, the scattered radiation is represented by arrows 39. The scatteredradiation 39 is imaged onto the CCD camera 37, and indicates that theflow of the imprintable material 33 has not been completed.

The reason why the scattering occurs can be understood with reference tothe refractive index of the gas 42 as compared to the refractive indicesof the imprint template 31 and the imprintable material 33. A typicalrefractive index for the gas 42, which may be for example nitrogen orair, is 1. A typical refractive index of the imprint template 31, whichconventionally is made from quartz, is 1.9. A typical refractive indexof the imprintable material 33, which typically comprises siliconcontaining a monomer, is 1.6. Reflection of radiation occurs at theinterface between two materials when there is significant differencebetween the refractive indices of those two materials. Thus, the beam ofradiation 35 will undergo reflection when it reaches an interfacebetween the imprint template 31 and a gas bubble 42, and an interfacebetween the gas bubble 42 and the imprintable material 33. Because thereare several interfaces, and these are disposed at different angles, manyreflections will occur, and these give rise to the scattered radiation39. The scattering of radiation from the gas bubbles 42 is detected bythe CCD camera 37 (see FIG. 4). To avoid complicating FIG. 5 b, thepassage of the beam of radiation 35 into the gas bubbles 42 is notillustrated.

Referring to FIG. 5 c, once the flow of imprintable material 33 has beencompleted, the recesses 40 are substantially completely filled withimprintable material, and no or few gas bubbles are present. Since therefractive index of the imprint template 31 is close to the refractiveindex of the imprintable material 33, the amount of reflection whichoccurs at the interface between the imprint template and the imprintablematerial is low. The beam of radiation 35 passes through the interfacewithout being substantially disturbed; significant scattering does notoccur, and therefore little or no scattered radiation is detected by theCCD camera 37 (see FIG. 4). In some instances, a residual amount ofradiation may be scattered at the interface due to the relatively smalldifference between the refractive index of the imprint template and therefractive index of the imprintable material. However, this will besmall compared with the amount of radiation that would be scattered bythe presence of the gas bubbles.

In general, the degree of scattering detected by the CCD camera 37provides an indication of the number and/or size of gas bubbles 42located underneath the imprint template 31.

In an embodiment, a measurement of the point in time at which the flowof imprintable material 33 into recesses 40 of the imprint template 31has been completed may be made. This measurement may be doneautomatically, for example by analyzing the total intensity of the imagedetected by the CCD camera 37, or the contrast of the image seen by theCCD camera. This may allow imprint lithography to be performed moreefficiently, since the imprint template 31 may be moved to thedisengaged position immediately once the flow of imprintable material 33has been completed. In typical imprint lithography, it may not be knownwhen the flow of imprintable material had been completed, with theresult that the imprint template remained in the imprint position forlonger than was necessary. Thus, a reduction in the time required toperform imprint lithography, correspondingly an increase in theproductivity of imprint lithography, may be achieved.

Further, in an embodiment, an indication of the presence of a defect inan imprinted pattern may be provided. For example, it may be the casethat bubbles are trapped beneath the imprint template, and have notmigrated out from under the template within a maximum time allowed forthe flow of imprintable material. These bubbles may cause defects in theimprinted pattern. By detecting the presence of the bubbles, anembodiment allows a defective imprinted pattern to be identified. Thedefective imprinted pattern may be, for example, reworked or discarded.In conventional imprint lithography systems, defective patterns may notbe identified until manufacture of a device has been completed, and thedevice tested.

Following the completion of the flow of the imprintable material 33, theimprint template 31 is returned to the disengaged position. Theimprintable material 33 is then illuminated with UV radiation in orderto activate a chemical reaction which fixes the imprinted pattern intothe imprintable material 33 (i.e. the imprintable material is cured). Inan embodiment, the CCD camera 37 and condenser lens 36 are movable inthe x and/or y directions, and are moved away from the imprint template31 before illumination by UV radiation takes place. In an embodiment,the CCD camera 37 and/or condenser lens 36 are transparent to UVradiation, and the UV radiation is directed through one or both of themto the imprintable material 33. In an embodiment, the UV radiation maybe directed at the imprintable material 33 at an angle, so that the CCDcamera 37 does not need to move or to be transparent to UV radiation. Inan embodiment (not illustrated), the CCD camera may be placed such thatit views the imprint template from an angle instead of beingsubstantially perpendicular, to allow substantially perpendicularillumination of the imprint template by the UV radiation.

As mentioned above, the output of the CCD camera 37 may be analyzedautomatically. This may be done, for example, by adding together theentire output of the CCD camera 37 to provide a single intensity value.A controller C may be arranged to compared this intensity value with athreshold value, and when the single intensity value falls below thethreshold value this may be interpreted as indicating that the flow ofimprintable material 33 has been completed. The controller C will thenprovide an output signal indicating that the imprint template 31 may bemoved to the disengaged position. It will be appreciated that for thisarrangement the CCD camera 37 may be replaced by a single large areaphotodiode.

In an arrangement, the controller C may be used to analyze the contrastof the image detected by the CCD camera 37 (i.e. the difference inintensity seen by different pixels of the camera). Radiation scatteredfrom gas bubbles will be unevenly distributed and will not provide aneven intensity over the CCD camera 37. This means that the amount ofcontrast is an indication of the number and/or size of gas bubbles 42that are trapped beneath the imprint template 31. Once the scatteringhas been eliminated, or substantially reduced, then what will be left issome background illumination of the CCD camera 37, which will besubstantially uniform. Thus, when the contrast falls below a thresholdvalue, this may be interpreted as indicating that the flow ofimprintable material 33 has completed. The controller C will thenprovide an output signal indicating that the imprint template 31 may bemoved to the disengaged position.

In general, it may be the case that not every gas bubble 42 can beeliminated by the flow of imprintable material 33. The threshold maytherefore be set to a level such that it indicates that the majority ofgas bubbles 42 have been eliminated, or a value which is found tocorrespond with imprinted patterns that have the required form (i.e.that are not overly damaged by the presence of gas bubbles).

Referring to FIG. 6, an imprint lithography apparatus according to anembodiment of the invention is schematically depicted. The imprintlithography apparatus comprises a substrate table 30 and a templateholder (not shown for ease of illustration and clarity) holding animprint template 31. A substrate 32 is held on the substrate table 30and is provided with a region of imprintable material 33. The imprinttemplate 31 is movable in the z-direction. A LED with a radiation output34 is provided to one side of the imprint template 31, and is arrangedto emit radiation at a wavelength of 400 nanometers in the direction ofa beam splitter 45. Alternatively or additionally, radiation may besupplied to a radiation output 34 from a remote radiation source. Thebeam splitter 45 is arranged to direct radiation from the LED 34 towardsthe imprint template 31. A condensing lens 36 is located above the beamsplitter 45 and is arranged to focus an image of the imprint template 31onto a CCD camera 37.

In use, once the imprint template 31 has been moved to the imprintposition, the LED 34 is switched on and directs a beam of radiation 46via the beam splitter 45 at an upper surface of the imprint template.When gas bubbles are present between the imprint template 31 and theimprintable material 33, scattering of the beam 46 will occur, asillustrated by arrows 47. In general, the scattered radiation 47 is notcondensed by the condenser lens 36, and therefore is not detected by theCCD camera 37.

Once the flow of imprintable material 33, into recesses of the imprinttemplate 31 has been completed, there will be no or a reduced number ofgas bubbles present at the interface between the imprint template andthe imprintable material. The beam of radiation 46 will therefore nolonger be substantially scattered, but will instead be reflected fromthe upper surface of the substrate 32, as shown by arrow 48. Thereflected radiation is condensed by the condenser lens 36, and isdetected by the CCD camera 37.

The presence of, and amount of, radiation detected at the CCD camera 37provides an indication of whether the flow of imprintable material 33into recesses of the imprint template 31 has been completed. Themeasurement provided by the CCD camera 37 is opposite to that providedin the embodiment illustrated in relation to FIGS. 4 and 5, in the sensethat the presence of gas bubbles reduces the amount of radiationdetected by the CCD camera 37 (in the previously described embodimentthe presence of gas bubbles increased the amount of detected radiation).Thus, the CCD camera 37 may be interpreted as indicating that the flowof imprintable material 33 has been completed when the total intensityoutput from the CCD camera 37 rises above a threshold value. When theoutput from the CCD camera 37 rises above the threshold value, acontroller C provides an output signal indicating that the imprinttemplate 31 may be moved to the disengaged position. Where a singletotal intensity output is used, the CCD camera 37 may be replaced by asingle large area photodiode.

In an arrangement, the contrast of radiation detected by the CCD camera37 may be used to obtain an indication of when the flow of imprintablematerial 33 has been completed. Reflection of the beam of radiation 46from the surface of the substrate 32, together with reflection which mayalso be seen from the surface of the imprint template 31, will lead tosubstantially uniform illumination of the CCD camera 37 (i.e. thecontrast will be low). However, during flow of the imprintable material33, when gas bubbles are present between the imprint template 31 and theimprintable material 33, the majority of radiation incident on theimprint template and the imprintable material will be scattered, and theamount of radiation that is reflected will be small. Although most ofthe scattered radiation is not incident on the CCD camera 37, the smallamount that is incident on the CCD camera is significant compared to theamount of reflected radiation incident on the CCD camera. Thus, thescattered radiation will give rise to an uneven distribution ofradiation on the CCD camera 37, i.e. a significant amount of contrast.When the flow of imprintable material 33 has been completed, the amountof reflected radiation is greater, and the amount of scattered radiationis reduced, and this is seen as a reduction of the contrast of radiationincident on the CCD camera 37. Thus, when the contrast seen by the CCDcamera 37 falls below a threshold value, this may be interpreted by thecontroller C as indicating that the flow of imprintable material 33 hasbeen completed. The controller C will then provide an output signalindicating that the imprint template 31 may be moved to the disengagedposition.

A proportion of the beam of radiation 46 will pass through the beamsplitter 45, and a proportion of the beam of radiation 48 will bereflected by the beam splitter. The unwanted beams are directed at beamstops (not illustrated) which are arranged to absorb the beams withoutreflecting them. In some instances suitable polarizing plates may beused in conjunction with a polarizing beam splitter to minimize theamount of energy in the unwanted beams, or even to substantiallyeliminate the unwanted beams.

In the described embodiments, for ease of illustration the beam ofradiation 35, 46 is shown as only intersecting a small portion of theimprint template 31. It will be appreciated that in practice the beam ofradiation 35, 46 may be sufficiently broad to illuminate the entireimprint template. In some instances it may be desired to illuminate andmeasure only a part of the imprint template 31. Where this is done, themeasurement may be interpreted as being representative of the entireimprint template 31.

In the described embodiments, the output of the CCD camera 37 iscompared with a threshold value for the entire surface of the CCDcamera. It will be appreciated that in an arrangement, the output of theCCD camera may be considered as set of adjoining areas (for example nineequally sized rectangles). Where this is done, the output for each areamay be separately compared with a threshold value. This provides spatialinformation regarding the location of areas in which the flow ofimprintable material is incomplete. This information may be used toimprove the imprint process. For example, once an area for which theflow of imprintable material is incomplete has been located, extraimprint pressure may be applied to that area.

Although the LED 34 is arranged to emit radiation at 400 nanometers,other suitable wavelengths or bands of wavelengths may be used.Furthermore, radiation sources other than an LED may be used. Ingeneral, the wavelength used should be long enough that it will notcause curing of the imprintable material, but short enough that it willscatter from small gas bubbles (i.e. bubbles that are significantlysmaller than the size of features on the imprint template). A suitablerange of wavelengths of radiation is 400 to 700 nanometers. In anembodiment, infrared radiation is not used, as this may be absorbed bythe imprintable material or the substrate and cause unwanted heating.

The described embodiments may be used in connection with conventionalimprintable material, for example a photopolymerizable material such asa mixture of monomers, an initiator and silicon. Typically the monomermay comprise for example acrylate or methacrylate. If required, therefractive index of the imprintable material may be increased byincreasing the proportion of silicon in the material. This will reducethe refractive index difference between the imprint template 31 and theimprintable material 33, thereby improving the operation. The refractiveindices of the imprint template 31 and the imprintable material 33 maybe adjusted so that they are matched (i.e. substantially identical). Ananother way of reducing the refractive index difference between theimprint template 31 and the imprintable material 33 is to make theimprint template from a material having a lower refractive index. Onematerial that could be used to do this is plastic. However, plastic maybecome damaged over time due to exposure to ultraviolet radiation.

In order to increase the proportion of the beam of radiation 35, 46 thatprovides a useful signal, an anti-reflection coating for the wavelengthof the beam of radiation may be provided on an uppermost surface of theimprint template 31.

While specific examples of the invention have been described above, itwill be appreciated that the invention may be practiced otherwise thanas described. The description is not intended to limit the invention.

1. A lithographic apparatus, comprising: a template holder configured to hold an imprint template; a substrate table arranged to receive a substrate; a detector configured to detect radiation scattered from an interface between the imprint template and imprintable material provided on the substrate when at least part of the template is in contact with the imprintable material; and a controller configured to compare the contrast of radiation incident on the detector with a threshold, and generate an output when the contrast falls below the threshold.
 2. The apparatus according to claim 1, wherein the detector comprises a charge coupled device (CCD) camera or one or more photodiodes.
 3. The apparatus according to claim 1, further comprising a radiation output configured to direct UV radiation at the imprintable material at an angle, such that the UV radiation is not incident upon the detector.
 4. The apparatus according to claim 1, wherein the detector is translatable.
 5. The apparatus according to claim 1, wherein the detector is arranged to generate a plurality of outputs, each output representing a different area of the detector.
 6. The apparatus according to claim 1, further comprising a radiation output arranged to illuminate a part of the imprint template and arranged to supply radiation at a wavelength or a band of wavelengths within the range 400 to 700 nanometers.
 7. The apparatus according to claim 1, further comprising a radiation output arranged to illuminate a part of the imprint template and arranged to direct radiation at the imprint template such that the radiation is incident at an angle upon the imprint template, the angle being sufficiently large that radiation reflected from the substrate is not incident upon the detector.
 8. The apparatus according to claim 1, wherein the refractive index of the imprintable material and the refractive index of the imprint template are substantially matched.
 9. The apparatus according to claim 1, comprising a beam splitter located between the detector and the imprint template.
 10. The apparatus according to claim 1, wherein an uppermost surface of the imprint template is provided with an anti-reflection coating which is effective at the wavelength or band of wavelengths of radiation emitted from the radiation output.
 11. A method of imprint lithography, comprising: pressing an imprint template onto a layer of imprintable material on a substrate; illuminating a part of the imprint template with radiation; and detecting radiation scattered from an interface between the imprint template and the imprintable material on the substrate to determine whether the imprintable material has flowed into a recess of the imprint template; and comparing a contrast of the detected radiation with a threshold, and generating an output when the contrast falls below the threshold.
 12. The method according to claim 11, wherein the refractive index of the imprintable material and the refractive index of the imprint template are substantially matched.
 13. The method according to claim 11, wherein the illuminating comprises directing UV radiation at the imprintable material at an angle, such that the UV radiation is not incident upon a detector used for the detecting.
 14. The method according to claim 11, further comprising moving a detector used for the detecting.
 15. The method according to claim 11, wherein the illuminating comprises directing radiation at the imprint template such that the radiation is incident at an angle upon the imprint template, the angle being sufficiently large that radiation reflected from the substrate is not incident upon a detector used for the detecting.
 16. The method according to claim 11, wherein an uppermost surface of the imprint template is provided with an anti-reflection coating which is effective at the wavelength or band of wavelengths of radiation emitted during the illuminating. 